1. Field of the Invention
The present invention relates to a MOS transistor and, more particularly, to a vertical MOS transistor and a method of forming the transistor.
2. Description of the Related Art
A MOS transistor is a well-known element that is one of the fundamental building blocks of many electrical circuits. There are two basic types of MOS transistors, a p-channel or PMOS transistor and an n-channel or NMOS transistor. A PMOS transistor has p+ source and drain regions and a p-channel when conducting, while a NMOS transistor has n+ source and drain regions and an n-channel when conducting.
FIG. 1 shows a cross-sectional view that illustrates one example of a conventional NMOS transistor 100. As shown in FIG. 1, transistor 100, which is formed in a p-type semiconductor material 110, such as a substrate or well, has spaced-apart n+ source and drain regions 112 and 114 that are formed in material 110.
In addition, transistor 100 has a channel region 116 that is located between source and drain regions 112 and 114. Further, transistor 100 includes a layer of gate oxide 120 that is formed over channel region 116, and a polysilicon gate 122 that is formed on gate oxide layer 120 over channel region 116.
In operation, material 110 and source region 112 are often connected to ground when drain region 114 is connected to a positive voltage source, such as 1.2V. As long as the voltage on gate 122 remains below a threshold voltage, substantially no charge carriers flow from source region 112 to drain region 114 (a small leakage current may be present). However, when the voltage on gate 122 equals or exceeds the threshold voltage, transistor 100 turns on and electrons begin to flow from source region 112 to drain region 114.
FIG. 2 shows a cross-sectional view that illustrates a second example of a conventional NMOS transistor 200. NMOS transistor 200 is similar to NMOS transistor 100 and, as a result, utilizes the same reference numerals to designate the structures which are common to both transistors.
As shown in FIG. 2, transistor 200 differs from transistor 100 in that material 110 is surrounded by an isolation region 210. In addition, material 110 is not connected to an external bias, such as a substrate or well contact and, as a result, electrically floats. Further, transistor 200 operates the same as transistor 100.
One of the limitations of transistors 100 and 200 is that the channel lengths of transistors 100 and 200 (the shortest distance between source and drain regions 112 and 114 at the surface of material 110) are defined by the minimum photolithographic feature size that is provided by the semiconductor fabrication process.
FIGS. 3A–3C show cross-sectional views that illustrate a MOS structure 300 during a conventional MOS transistor fabrication process. As shown in FIG. 3A, MOS structure 300 has a p-type semiconductor material 310, such as a substrate or well, and a layer of gate oxide 312 that is formed over material 310.
In addition, MOS structure 300 has a layer of polysilicon 314 that is formed on gate oxide layer 312, and a mask 316 that is formed on a portion of polysilicon layer 314. As further shown in FIG. 3A, mask 316 has a length L1 that is equal to the minimum feature size provided by the fabrication process.
Following the formation of MOS structure 300 in FIG. 3A, structure 300 is anisotropically etched until the exposed regions of polysilicon layer 314 have been removed from the surface of gate oxide layer 312. As shown in FIG. 3B, the etch forms a gate 318 that has a gate length L2 that is defined by the length L1 of mask 316. Following this, mask 316 is removed.
Next, as shown in FIG. 3C, structure 300 is implanted with an n-type dopant to form source and drain regions 320 and 322. Source and drain regions 320 and 322 can be single heavily-doped n+ implanted regions, or can be lightly-doped n− LDD regions. As further shown in FIG. 3C, the implant defines a channel 324 that has a channel length L3 that is defined by the length L2 of gate 318. (Current-generation low temperature annealing and activating processes allow very little lateral diffusion of the dopants.)
As a result, the channel length L3 is defined by the length L1 of mask 316 which has the minimum photolithographic feature size that is provided by the fabrication process. Thus, there is a need for a MOS transistor and a method of forming the transistor that allow a channel length to be formed that is smaller than the minimum photolithographic feature size that is provided by the fabrication process.